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Why are police sirens different?

Emergency services – police, fire and ambulance – use sirens to let other road users know they are coming. These road users could have music blaring, windows up, or a helmet on, etc. So, they need to be loud and have a variable pitch to help with localising where the siren is coming from.

The traditional UK siren is a two-tone “nee-naw” siren which has mostly been replaced now with American-style wail, yelp or phaser sirens. This original two-tone sound was created by two different horns operated alternately. New sirens use one speaker (or two speakers playing the same sound). These sirens typically operate between 1kHz and 3kHz as this is where our ears are the most sensitive. I.e. the siren doesn’t have to have so much power to make itself heard because our ears hear sounds in that frequency better.

If you play middle A on a piano (the one above middle C), then go an octave above that, that’s almost 1kHz. 3kHz is another octave and a half above there. Our ears have trouble figuring out where sounds are coming from that are in the 2-4kHz range.

Different types of siren sound are useful in different applications. The generic siren wail (a slow rise and fall of the pitch) is very easy for people to hear from a long way away, but it’s hard for people to know which direction it’s coming from. Police will move to yelp or phaser with faster oscillation or use an electronic digital buzz sound containing many more frequencies which is more easy to locate.

How loud are police sirens?

Emergency services sirens are typically around 110 decibels (dB).

Long term use of sirens is damaging to the driver of the vehicle as sound levels have been measured at up to 120dB (decibels) and obviously will affect the drivers of the emergency services vehicles. At this level, you get your entire day’s worth of noise energy in less than 15 minutes. 120dB is 32 times louder than a vacuum cleaner. Motorcycle police officers are known to wear earplugs not only for this, but also for the fact that their bikes are loud, too.

How do we tell where a siren is coming from?

Our ears have four built-in ways of localising sound.

  • Precedence or Haas effect – your brain filters out any echoes and just uses the initial sound
  • Interaural phase difference – sound hitting one ear will be in a different phase when it hits the second ear.
  • Interaural time difference – sound hits the ear furthest away from the source later than the ear closest to the source.
  • Interaural amplitude difference – sound hitting the ear closest to the source is louder than at the ear further away.

Problems occur when we start hearing the echoes off buildings rather than the direct sound as this can make a siren seem like it’s coming from a different direction. Also, if a siren is directly in front of you, your brain may perceive it being behind you.

Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.

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